Question

Hydrogen halides of the highest$\mathrm{Pka}$ is?

Answer 1

Step 1: Acidity order of Hydrogen halides:

The Hydrogen halides are given as; $\mathrm{HF},\mathrm{HCl},\mathrm{HBr},\mathrm{HI}$.

This trend is accountable for the fact that the acidity strength is denoted as the tendency to donate ${\mathrm{H}}^{+}$ions in the solution.

Hence the acidity strength of the hydrogen halides inversely depends upon the strength of the $\mathrm{H}-\mathrm{X}$ bond.

As the $\mathrm{H}-\mathrm{X}$ bond strength of the Hydrogen halides is given in the order: $\mathrm{HI}<\mathrm{HBr}<\mathrm{HCl}<\mathrm{HF}$.

Thus, the order of acidic strength of hydrogen halides is given as;

$\mathrm{HI}>\mathrm{HBr}>\mathrm{HCl}>\mathrm{HF}$

Step 2: Relation between acidity and ${\mathrm{pK}}_{\mathrm{a}}$ value:

${\mathrm{pK}}_{\mathrm{a}}$ is denotes as the negative logarithm of the acid dissociation constant ${\mathrm{K}}_{\mathrm{a}}$ in the solution, which can be given as:

${\mathrm{pK}}_{\mathrm{a}}=-\log \left[{\mathrm{K}}_{\mathrm{a}}\right]$

The acid dissociation constant ${\mathrm{K}}_{\mathrm{a}}$ basically denotes the acidic strength of any acid.

As, the ${\mathrm{pk}}_{\mathrm{a}}$ value is inversely proportional ${\mathrm{K}}_{\mathrm{a}}$, hence the value of ${\mathrm{pk}}_{\mathrm{a}}$decreases with the increase in the value of the acidic strength.

Step 3: Hydrogen halides of the highest ${\mathrm{pK}}_{\mathrm{a}}$:

Thus, the order of acidic strength of hydrogen halides is given as;

$\mathrm{HI}>\mathrm{HBr}>\mathrm{HCl}>\mathrm{HF}$

Thus, the ${\mathrm{pk}}_{\mathrm{a}}$value order of the given hydrogen halides will be in the order:

$\mathrm{HI}<\mathrm{HBr}<\mathrm{HCl}<\mathrm{HF}$

Hence, we can conclude from the above order that $\mathrm{HF}$ has the highest ${\mathrm{pK}}_{\mathrm{a}}$ value.

Thus, the hydrogen halide of the highest ${\mathrm{pK}}_{\mathrm{a}}$ is $\mathrm{HF}$.